1. Field of the Invention
The present invention relates to the field of free piston engines.
2. Prior Art
Various types of free piston engines are well known in the prior art. Of particular relevance to the present invention are the free piston engines and methods disclosed in U.S. Patent Application Publication No. 2011/0083643, the disclosure of which is hereby incorporated by reference. Those engines utilize a high pressure hydraulic rail and a low pressure hydraulic rail and a plurality of hydraulic pistons and valving to controllably couple the hydraulic pistons to the high pressure hydraulic rail or the low pressure hydraulic rail. In each cylinder a central hydraulic piston is connected to the free piston and configured so as to draw the free piston away from the top dead center position, such as during an intake stroke, or to exert a force on the free piston toward the top dead center position, such as during a compression stroke or a power stroke during which hydraulic energy is delivered to the high pressure rail. The additional hydraulic pistons are symmetrically distributed around the center hydraulic piston and may be controllably coupled to the high pressure rail or the low pressure rail as appropriate for a compression stroke, and the output of hydraulic energy to the high pressure rail during a power stroke as appropriate to control the free piston velocities, excursion, etc.